The bodies are so scattered across the Canadian wilderness that they often decay, or are devoured by scavengers, long before wildlife biologists can find them.
Because of that, researchers face a tough challenge trying to piece together clues to what is causing so many North American moose to die. Population numbers are down by 70 per cent in some areas and nobody knows why, although climate change, increased predation and unknown diseases have emerged as possible suspects in the moose mystery.
Wildlife biologists are hoping a major new study being launched in British Columbia this winter – in which 230 moose will be collared and tracked by satellite – will provide vital information on why the big ungulates are perishing. The project will use Global Positioning System technology to track moose over a vast area of the province, with teams ready to respond as soon as a "mortality sensor" sends an alarm.
"The collar will indicate that the moose is down. The collar will say: 'I am dead,'" said Rodger Stewart, resource management director for B.C.'s Ministry of Forests, Lands and Natural Resources. "We'll be able to track its location. We'll be able to arrive there, ideally within 48 hours basically, and we'll be able to get a hand on what's happening." To determine where and why moose are dying, he said, researchers need to get to a body while it is still fresh and perform a necropsy.
There are an estimated 800,000 to 1.2 million moose in North America. About 170,000 live in B.C., more than in any other area. The overall population decline isn't known, but game surveys show drops of between 20 per cent and 70 per cent in sections of B.C.'s Cariboo region alone.
Mr. Stewart said moose will be tracked in five sprawling regions of B.C., and researchers will respond by helicopter if necessary when a death is signalled. GPS technology allows researchers to get data on collared animals at intervals as frequently as every minute, though daily signals are usually adequate, and to locate them within metres, even in dense bush.
Typically used by biologists to study wildlife movement and habitat use, GPS tracking may now prove most valuable for its ability to signal death. The technique is already in use in Minnesota, where 110 moose have been collared. The moose population in that state has fallen so dramatically – by 70 per cent since 2006 – that all moose hunting has been banned.
Research there has suggested wolf predation is higher than expected, but scientists are still struggling to understand "tip-over events" in which the moose simply lie down and die from unknown causes.
Mr. Stewart said the situation in B.C. is not as stark as in Minnesota, where the population decline is statewide, but the fear is Minnesota is signalling a trend that may soon sweep across the entire range of North American moose. In B.C. and several other provinces, moose are declining in some areas but stable in others, and no clear cause is emerging.
"That's the frustration at this point – that we cannot point to any particular factor as having a driving influence on the moose population," Mr. Stewart said. "We're in circumstances where we have a number of allegations about what's driving the decline, but there's no material evidence. And I think from the anecdotes we're getting … that there are similar issues arising in the moose range across North America."
Moose populations don't seem to be dropping in more northerly regions across the continent, however, which has led to speculation that Minnesota is the leading edge of a decline brought on by climate change. But another theory is that moose populations may be naturally falling back after a period of rapid expansion.
"We can make all kinds of speculation about it, but we also have to remind ourselves that the moose appeared on the land base here suddenly, in terms of geologic time anyway, around the time of the First World War," Mr. Stewart said. "Moose are to some degree strangers in these ecosystems."
In the Cariboo region of central B.C., for example, moose spread across the landscape only in the past century. "So … is it possible that we're seeing the edge of some form of natural ecological oscillation?" he asked.
Doug Heard, a wildlife biologist with the province, said researchers have been able to rule out some suspects. There is no correlation between numbers of moose and the amount of forest killed by mountain pine beetles. Hunting pressure has remained relatively unchanged for 30 years and can't be to blame. There is no sign of disease, so far, but Mr. Heard says researchers seldom find moose that have died in the bush. The GPS study is expected to change that.
"We will get the causes of death – hopefully for all of them," he said of the moose to be collared. "It seems to be this plethora of causes, [and] possibly climate change has induced stress that may make them more vulnerable. That being said, we don't really know."
Ronald Moen, a research associate at the Natural Resources Research Institute at the University of Minnesota, said the problem appears to be bigger than North America. "In Sweden they are seeing similar issues … the question is, how broad is the problem?" he said recently, just before boarding a flight to Europe.
He was on his way to a conference of wildlife scientists from Sweden, the United States, Finland, Norway and Russia who all want to know what's causing the moose collapse.
Dr. Moen, who has warned that Minnesota could lose its moose by 2020, feels there is a link to climate change. "There's something going on clearly across the southern edge [of moose range in North America]," he said. "It's correlated with warmer temperatures."